Don't get me wrong: out of all the series on the roster, FE has given me some of the fondest of memories. But this "everybody's happy scenario" where we bring everyone back every time and also keep adding the most recent protags approaches unhealthy and irresponsible resource direction.
I mean, in Three Houses case, you either were going to get Byleth, Edelgard, Dimitri or Claude, all of whom are main characters. Part of Smash Ultimate was also about picking fan-favourite characters and if you look at the 1st Party DLC picks, Pyra & Mythra were very popular, Min-Min was the most popular ARMS character and all 4 Three Houses main characters, including Byleth, are popular in their franchise's fanbase.
Corrin was very much chosen for their dragon abilities and dragons and manaketes are a major part of Fire Emblem. And fans have certainly asked for more elements to be represented for Fire Emblem in Smash Bros. And compared to other manaketes, Corrin was the only one who could manifest parts of their dragon body for attacks and whatnot, whereas other Manaketes couldn't or could only manifest wings.
Not to mention Corrin's scenario was either they got in or they don't get in. Smash 4's DLC was the only chance Corrin was ever going to get in, basically the opposite of ARMS and Xenoblade 2's scenarios where they were too early to get in base game. In Fates' case, it had already come out in Japan around half-a-year before their DLC was announced.
And we don't even know if there would've been another character who would've taken Corrin's place had they not been chosen. There was no guarantee that Echoes was going to get a character for Ultimate if Corrin wasn't in beforehand and there's no guarantee another character would've been in their DLC spot. After Cloud, it could've been only Bayonetta. Sakurai has not made a mention of any other characters looked at for DLC, including Smash 4 DLC.
So it's quite possible it's either we got Corrin or we didn't get anybody, much like how not having Dr. Mario, Dark Pit and Lucina wouldn't suddenly give us other characters.